1. Field of the invention
The present invention concerns an electron gun for a cathode ray tube, especially for color television.
A cathode ray tube is constituted by a glass envelope under vacuum presenting at its rear portion a plate-screen the internal face of which is covered with cathodoluminescent material (called luminophorous material) emitting a visible light radiation when they are excited by an electron beam produced by one or several electron guns. For visualization in color the screen is generally formed of triple lines of luminophosphoruses, one line for each primary color, normally red, green and blue. In a color visualization tube of the shadow mask type, three electron guns are foreseen, each being intended to excite a particular color, the perforated mask disposed before the screen allowing the electron beam foreseen for one color only excites the light-emitting or luminophor particles of that color.
An electron gun is formed, on the one hand, of a cathode having a surface emitting electrons when it is heated by a filament and, on the other hand, of electrodes or grids generally designated G.sub.1, G.sub.2, G.sub.3, etc, the index representing the position with respect to the cathode, this index increasing with the distance away from the cathode of which the first two G.sub.1 and G.sub.2 performing a prefocalization of the electron beam and the following G.sub.3, G.sub.4, etc. form electrostatic lenses for focussing and concentrating the electron beam. The modulation of the electron beam as a function of the intensity of the point or dot to be projected onto the screen is obtained through the modulation of the voltage of the cathode or by the modulation of the voltage of the first grid G.sub.1 (Wehnelt).
The electrodes of the electrostatic grids are normally brought to very high voltages, most frequently of about several kilovolts. The invention concerns more particularly a tube, at least one grid of which of the electrostatic lenses is, during normal working, brought to a very high voltage, of about 25 kilovolts, for example, and which retains a voltage of the same value over a long period of time after the shut-down of the supply of this tube. This voltage does not disappear immediately for the following reason: the glass envelope has an internal conductive coating to which is connected the said grid as well as an external conductive coating brought to a voltage of the mass; the glass envelope thus constitutes, with its coatings, a large-capacity condensor that retains its load over a long period even after it has been disconnected from its power supply; due to this fact, the internal coating and the electrode to which it is connected maintain the high voltage.
It has been noted that the high voltage maintained on the grid could provoke - when the distance separating this grid from the cathode is relatively small - the projection of electrons on the center of the screen, being apparent by one or several visible luminous patches whereas the temperature of the cathode remains high. Such a defect is sometimes called "the cathode glimmer".
In order to overcome this drawback, it has been proposed to dispose in the circuit outside the tube discharge diodes. But this solution remains costly.